Scaling of feeding biomechanics in the horn shark Heterodontus francisci: ontogenetic constraints on durophagy

Organismal performance changes over ontogeny as the musculoskeletal systems underlying animal behavior grow in relative size and shape. As performance is a determinant of feeding ecology, ontogenetic changes in the former can influence the latter. The horn shark Heterodontus francisci consumes hard-shelled benthic invertebrates, which may be problematic for younger animals with lower performance capacities. Scaling of feeding biomechanics was investigated in H. francisci (n=16, 19–59 cm standard length (SL)) to determine the biomechanical basis of allometric changes in feeding performance and whether this performance capacity constrains hard-prey consumption over ontogeny. Positive allometry of anterior (8–163 N) and posterior (15–382 N) theoretical bite force was attributed to positive allometry of cross-sectional area in two jaw adducting muscles and mechanical advantage at the posterior bite point (0.79–1.26). Mechanical advantage for anterior biting scaled isometrically (0.52). Fracture forces for purple sea urchins Strongylocentrotus purpuratus consumed by H. francisci ranged from 24 to 430 N. Comparison of these fracture forces to the bite force of H. francisci suggests that H. francisci is unable to consume hard prey early in its life history, but can consume the majority of S. purpuratus by the time it reaches maximum size. Despite this constraint, positive allometry of biting performance appears to facilitate an earlier entry into the durophagous niche than would an isometric ontogenetic trajectory. The posterior gape of H. francisci is significantly smaller than the urchins capable of being crushed by its posterior bite force. Thus, the high posterior bite forces of H. francisci cannot be fully utilized while consuming prey of similar toughness and size to S. purpuratus, and its potential trophic niche is primarily determined by anterior biting capacity.     

Stomach content analysis of juvenile, scalloped hammerhead shark Sphyrna lewini captured off the coast of Mazatlán, Mexico

We quantified the diet of juvenile, scalloped hammerhead shark Sphyrna lewini in the area off Mazatlan, Sinaloa, Mexico, to understand their feeding ecology this shark. The prey species of Sphyrna lewini were identified and quantified from stomach content analysis. In addition, we determined the variations between genders. During two fishing seasons (2000–2001 and 2001–2002), we analyzed 232 stomachs, of which 85% contained food. The trophic spectrum was composed of three species of cephalopods, six of crustaceans and 19 species of fish from mainly pelagic and benthic habitats. According to the Index of Relative Importance (%IRI), the cephalopod Loliolopsis diomedeae with IRI = 18%, fish of the family Carangidae IRI = 25% and family Synodontidae IRI = 19% constituted the main prey in general. The trophic niche width was <0.4, which indicated that S. lewini juveniles in this area feed on a wide range of prey items, though they showed a preference for a few prey items.     

Food habits of the silky shark Carcharhinus falciformis (Müller & Henle, 1839) off the western coast of Baja California Sur,Mexico

The objective of this study was to establish the trophic niche of the silky shark and to determine the ecological role of this predator in the ecosystem close to Baja California. The trophic spectrum was analyzed from samples taken during summer and autumn (2000–2002) from the fishing camps of Punta Lobos and Punta Belcher on the western coast of Baja California Sur. A total of 263 stomach contents were analyzed (143 with food; 120 empty). The index of relative importance (IRI) showed that at Punta Lobos, silky sharks fed mainly on red crabs Pleuroncodes planipes (%IRI = 83%), whereas at Punta Belcher the main food item was the jumbo squid Dosidicus gigas (%IRI = 41%), followed by chub mackerel Scomber japonicus (%IRI = 33%). According to the Levin Index (Bi), the trophic niche breadth in silky sharks is low (Bi = <0.6), which means that silky sharks are specialist predators because they mainly consume three prey types: red crab, chub mackerel, and jumbo squid. The Shannon‐Wiener Index indicated that all trophic categories at Punta Belcher (0.85–1.22) had lower diversity than at Punta Lobos (0.50–1.6), because the silky shark feeds more on tropical prey found close to Punta Lobos. The Morisita‐Horn Index (Cλ) showed an overlap in the diet between the two areas analyzed and between sexes (Cλ = >0.6). The juveniles and adult females did not show any overlap. In the caloric analysis of the main prey, the jumbo squid (D. gigas) contributed the most calories to the silky shark diet (76%).     

Ontogenetic diet shifts of rough scad Trachurus lathami in the North Patagonian shelf (south-west Atlantic Ocean)

Rough scad Trachurus lathami is a key pelagic fish in the Argentinean continental shelf (ACS, south-west Atlantic Ocean), with recent increases in abundance. It is a main prey of fishes and marine mammals, and shares the environment with commercially relevant pelagic species (Engraulis anchoita and Scomber colias), playing an important role linking lower and upper trophic levels in the ecosystem. This study aims to determine the ontogenetic changes in the diet composition, feeding strategy, trophic niche breadth and trophic level of T. lathami in the North Patagonian Shelf (43°–45°30′S). The stomach contents of adult fish (n = 238) were analysed. The results suggest a clear ontogenetic shift in the diet at a size of ~190 mm. Smaller individuals (160–190 mm) were specialized on misidaceans, and showed the highest trophic level, while larger T. lathami (221–230 mm) consumed decapods (Peisos petrunkevitchi) and teleosts (eggs and larvae). Trophic niche breadth was higher at the medium-sized class (191–220 mm), which mainly preyed on copepods (Calanoides carinatus) and chaetognaths (Sagitta spp.), evidencing a more diverse diet and a rather generalist strategy. Updated information on the trophic ecology of T. lathami evidences its extremely plastic feeding behaviour, being able to adapt its trophic niche to the most readily available food items from the mesopelagic community.     

Trophic levels of teleost and elasmobranch species in the Persian Gulf and Oman Sea

Knowing the trophic level of marine organisms is essential to understanding their ecological role in the ecosystem and for quantifying the ecosystem effects of fishing to establish effective management of fishing resources. In comparison to other systems, information about the trophic level of marine organisms in the Persian Gulf and Oman Sea is very scarce. Here, the main aim was to estimate trophic level in these areas using all available diet information from different marine species using TrophLab software. The trophic level of 32 fish species was estimated with the available diet data. The trophic level ranged from 2.28 to 4.50. High trophic levels were found for Chorocentrus nudus (TL = 4.7), Saurida tumbil (TL = 4.6), Rhizoprionodon acutus (TL = 4.5), Torpedo sinuspersici (TL=4.5), Gymnura poecilura (TL = 4.5), Sphyraena putnamae (TL = 4.5) and Euthynnus affinis (TL = 4.5). In contrast, lower trophic levels were estimated for Tenualosa ilisha (TL = 2.28) and Sardinella sindensis (TL = 2.92). As expected, a positive correlation was found between the trophic level and body size, indicating changes in the diet due to variations in predatory capacities. The results of this study may be useful in the formulation of trophic indicators and modelling of the ecosystems.     

Isotopic niche differs between seal and fish‐eating killer whales (Orcinus orca) in northern Norway

Ecological diversity has been reported for killer whales (Orcinus orca) throughout the North Atlantic but patterns of prey specialization have remained poorly understood. We quantify interindividual dietary variations in killer whales (n = 38) sampled throughout the year in 2017–2018 in northern Norway using stable isotopic nitrogen (δ¹⁵N: ¹⁵N/¹⁴N) and carbon (δ¹³C: ¹³C/¹²C) ratios. A Gaussian mixture model assigned sampled individuals to three differentiated clusters, characterized by disparate nonoverlapping isotopic niches, that were consistent with predatory field observations: seal‐eaters, herring‐eaters, and lumpfish‐eaters. Seal‐eaters showed higher δ¹⁵N values (mean ± SD: 12.6 ± 0.3‰, range = 12.3–13.2‰, n = 10) compared to herring‐eaters (mean ± SD: 11.7 ± 0.2‰, range = 11.4–11.9‰, n = 19) and lumpfish‐eaters (mean ± SD: 11.6 ± 0.2‰, range = 11.3–11.9, n = 9). Elevated δ¹⁵N values for seal‐eaters, regardless of sampling season, confirmed feeding at high trophic levels throughout the year. However, a wide isotopic niche and low measured δ¹⁵N values in the seal‐eaters, compared to that of whales that would eat solely seals (δ_N‐measured = 12.6 vs. δ_N‐expected = 15.5), indicated a diverse diet that includes both fish and mammal prey. A narrow niche for killer whales sampled at herring and lumpfish seasonal grounds supported seasonal prey specialization reflective of local peaks in prey abundance for the two fish‐eating groups. Our results, thus, show differences in prey specialization within this killer whale population in Norway and that the episodic observations of killer whales feeding on prey other than fish are a consistent behavior, as reflected in different isotopic niches between seal and fish‐eating individuals.     

Effect of flood pulses on the trophic ecology of four piscivorous fishes from the eastern Amazon

This study investigated the effect of hydrological periods on the feeding activity and trophic interactions of four piscivorous fishes from the middle Xingu River, Brazil: pike‐characid Boulengerella cuvieri, dogtooth characin Hydrolycus armatus, dogtooth characin Hydrolycus tatauaia and South American silver croaker Plagioscion squamosissimus. Repletion Index (I_R%), Alimentary Index (I_Ai%) and food web properties were calculated for each species. A total of 825 specimens were collected. The I_R showed changes in feeding intensity of B. cuvieri, H. armatus and H. tatauaia among hydrological periods. Flood pulse showed no influence on composition and importance of food items consumed. Trophic connections showed that connectivity ranged from 0.025 to 0.038. The highest number of trophic connections (75) occurred in the high‐water period, when 51 food items were recorded and the lowest number of trophic connections (43) occurred in receding water, with 31 food items. In all food webs, over 45% of food items were consumed by only one species (ultra‐peripheral items), which is common in piscivorous fishes.     

Trophic niche of the Pacific Sierra (Scomberomorus sierra) in the southeastern Gulf of California: Assessing its importance as a predator and prey (Mesopredator) in the food web

The identification of interspecific links (trophic niche) is important to characterize resource use of a predator, and to know its trophic role (for example, mesopredator or top predator) in the food web. In this study, we examined: a) the trophic ecology of Scomberomorus sierra as a predator (niche breadth, trophic overlap, and trophic position) and b) its presence as a prey in the diets of the region´s top predators, to evaluate the critical link of S. sierra as a probable mesopredator in the food web of the southeastern Gulf of California (GC). Based on %PSIRI, the dominant diet of S. sierra were engraulids and cephalopods. The diet was similar between sexes and among size-classes. However, the isotopic niche breadth values and δ¹⁵N variance (>1) reflect a broad niche for young adults likely related to a) changes in morphology (e.g., size of the mouth), b) development of the visual system, and c) changes in the energy requirements of the species reproductive stages. Seasonal changes in prey species’ availability and abundance resulted in isotopic variations, indicating that S. sierra is an opportunistic predator. A wide range in trophic position value (from 3.8 to 4.2) indicated that it also is an intermediary carnivore, with a high degree of trophic plasticity. Although S. sierra has not a dominant role in top predators’ diets, they share some prey species such as anchovies and other fish, depending on predator size. Therefore, S. sierra is a species with many prey-predator relationships in the southeastern GC food webs that may be considered a critical trophic link. This information is crucial for an ecosystem-based fisheries management in the Gulf of California.     

Experimental assessment of trophic impacts from a network model of a seagrass ecosystem: Direct and indirect effects of gulf flounder, spot and pinfish on benthic polychaetes

Trophic cascades are predicted to occur when the abundance of predators is increased, directly reducing the abundance of the intermediate prey and indirectly increasing the abundance of the prey at the base of a food web. Mixed trophic impact analysis of a network model developed for Apalachee Bay, near St. Marks, FL, USA predicted such a trophic cascade, in that increased abundance of juvenile gulf flounder Paralichthys albigutta (x¯ = 149 mm SL, effective trophic level 3.9) should have a negative impact on juvenile spot Leiostomus xanthurus (x¯ = 30 mm SL, effective trophic level 2.9) and a positive impact on benthic polychaetes (effective trophic levels 2.3 for deposit feeders and 3.0 for predatory polychaetes) in Halodule wrightii seagrass beds. We tested the predictions of the mixed trophic impact analysis by manipulating the abundance of the high trophic-level species (juvenile gulf flounder) in a cage-exclusion study in the North River, near Harkers Island, NC, USA. We compared the polychaete communities in St. Marks, FL and Harkers Island, NC, and showed that they are 51% similar (Jaccard's Index) at the family level, with the same eight dominant families (Nereidae, Capitellidae, Syllidae, Spionidae, Cirratulidae, Terebellidae, Sabellidae, and Maldanidae) present in both locations. We used 24 open-bottom cages to enclose the benthos and its seagrass-associated animal communities. We manipulated each cage by assigning it to one of the following treatments: (1) inclusion of fishes in upper and intermediate trophic levels (1 juvenile gulf flounder and 10 juvenile spot, the flounder + spot treatment); (2) inclusion of the intermediate predator (10 juvenile spot with no gulf flounder, the spot-only treatment); and (3) no fish added (unmanipulated controls). Core samples taken within the cages provided pre- and post-experimental measures of polychaete density and biomass, and the difference in density and biomass were used as response variables. At the end of the experiment, we collected, weighed, and analyzed the gut contents of all juvenile spot present in the cages. Juvenile pinfish (Lagodon rhomboides, x¯ = 30 mm SL) were present at the end of the study, having arrived as larvae or being trapped during cage set-up, and these fish were also examined, because they also eat polychaetes and their natural densities exceeded our introduced spot densities. Significant differences among treatments were detected for the polychaete family Terebellidae for both the change in density and biomass (pre-experiment − post-experiment). Densities of the Terebellidae changed in the direction predicted by the network model's impact analysis, declining in the cages with spot added compared with the control cages. Analyses of the other response variables (post-experiment spot and pinfish densities and biomass, difference between pre- and post-experiment polychaete densities and biomass for other families, and post-experiment spot and pinfish stomach content biomass) showed no significant differences among treatments. Several variables (Nereidae densities, pinfish densities and biomass, and pinfish stomach content biomass) varied between cages with low and high seagrass cover (significant blocking effect, P < 0.001). Nereidae densities declined significantly in cages with high (73%) rather than with low coverage (31% cover) of seagrass. Pinfish density and biomass were significantly greater in the high seagrass cages at the end of the experiments (P < 0.001), suggesting that dense seagrass attracted them. We conclude that the high density of pinfish in dense seagrass was responsible for the decline in density of the Nereidae. The direct effect of intermediate predators (pinfish feeding on polychaete prey) can be influenced by preferential recruitment of fishes to structurally complex habitats. The direction of change of indirect effects, but not the magnitude, in multi-trophic-level food webs can be predicted by the mixed trophic impact analysis of network models. However, these indirect effects are likely to be small in magnitude relative to direct effects and may be difficult to detect experimentally, especially in low-power experimental caging studies with natural fluctuations in recruitment rates of competitor species.     

Climatically driven changes in primary production propagate through trophic levels

Climate and land‐use change are the major drivers of global biodiversity loss. Their effects are particularly acute for wide‐ranging consumers, but little is known about how these factors interact to affect the abundance of large carnivores and their herbivore prey. We analyzed population densities of a primary and secondary consumer (mule deer, Odocoileus hemionus, and mountain lion, Puma concolor) across a climatic gradient in western North America by combining satellite‐based maps of plant productivity with estimates of animal abundance and foraging area derived from Global Positioning Systems telemetry data (GPS). Mule deer density exhibited a positive, linear relationship with plant productivity (r² = 0.58), varying by a factor of 18 across the climate‐vegetation gradient (range: 38–697 individuals/100 km²). Mountain lion home range size decreased in response to increasing primary productivity and consequent changes in the abundance of their herbivore prey (range: 20–450 km²). This pattern resulted in a strong, positive association between plant productivity and mountain lion density (r² = 0.67). Despite varying densities, the ratio of prey to predator remained constant across the climatic gradient (mean ± SE = 363 ± 29 mule deer/mountain lion), suggesting that the determinacy of the effect of primary productivity on consumer density was conserved across trophic levels. As droughts and longer term climate changes reduce the suitability of marginal habitats, consumer home ranges will expand in order for individuals to meet basic nutritional requirements. These changes portend decreases in the abundance of large‐bodied, wide‐ranging wildlife through climatically driven reductions in carrying capacity, as well as increased human–wildlife interactions stemming from anthropogenic land use and habitat fragmentation.     

Reductions in the dietary niche of southern sea otters (Enhydra lutris nereis) from the Holocene to the Anthropocene

The sea otter (Enhydra lutris) is a marine mammal hunted to near extinction during the 1800s. Despite their well‐known modern importance as a keystone species, we know little about historical sea otter ecology. Here, we characterize the ecological niche of ancient southern sea otters (E. lutris nereis) using δ¹³C analysis and δ¹⁵N analysis of bones recovered from archaeological sites spanning ~7,000 to 350 years before present (N = 112 individuals) at five regions along the coast of California. These data are compared with previously published data on modern animals (N = 165) and potential modern prey items. In addition, we analyze the δ¹⁵N of individual amino acids for 23 individuals to test for differences in sea otter trophic ecology through time. After correcting for tissue‐specific and temporal isotopic effects, we employ nonparametric statistics and Bayesian niche models to quantify differences among ancient and modern animals. We find ancient otters occupied a larger isotopic niche than nearly all modern localities; likely reflecting broader habitat and prey use in prefur trade populations. In addition, ancient sea otters at the most southerly sites occupied an isotopic niche that was more than twice as large as ancient otters from northerly regions. This likely reflects greater invertebrate prey diversity in southern California relative to northern California. Thus, we suggest the potential dietary niche of sea otters in southern California could be larger than in central and northern California. At two sites, Año Nuevo and Monterey Bay, ancient otters had significantly higher δ¹⁵N values than modern populations. Amino acid δ¹⁵N data indicated this resulted from shifting baseline isotope values, rather than a change in sea otter trophic ecology. Our results help in better understanding the contemporary ecological role of sea otters and exemplify the strength of combing zooarchaeological and biological information to provide baseline data for conservation efforts.     

Quantitative prey species detection in predator guts across multiple trophic levels by mapping unassembled shotgun reads

Quantifying species trophic interaction strengths is crucial for understanding community dynamics and has significant implications for pest management and species conservation. DNA-based methods to identify species interactions have revolutionized these efforts, but a significant limitation is the poor ability to quantify the strength of trophic interactions, that is the biomass or number of prey consumed. We present an improved pipeline, called Lazaro, to map unassembled shotgun reads to a comprehensive arthropod mitogenome database and show that the number of prey reads detected is quantitatively predicted from the prey biomass consumed, even for indirect predation. Two feeding bioassays were performed: starved coccinellid larvae consuming different numbers of aphids (Prey Quantity bioassay), and starved coccinellid larvae consuming a chrysopid larvae that had consumed aphids (Direct and Indirect Predation bioassay). Prey taxonomic assignment against a mitochondrial genome database had high accuracy (99.8% positive predictive value) and the number of prey reads was directly related to the number of prey consumed and inversely related to the elapsed time since consumption with high significance (r² = .932, p = 4.92E-6). Aphids were detected up to 6 h after direct predation plus 3 h after indirect predation (9 h in total) and detection was related to the predator-specific decay rates. Lazaro enabled quantitative predictions of prey consumption across multiple trophic levels with high taxonomic resolution while eliminating all false positives, except for a few confirmed contaminants, and may be valuable for characterizing prey consumed by field-sampled predators. Moreover, Lazaro is readily applicable for species diversity determination from any degraded environmental DNA.     

Stable Isotope Evidence of Puma concolor (Felidae) Feeding Patterns in Agricultural Landscapes in Southeastern Brazil

We evaluated puma (Puma concolor) feeding patterns in southeastern Brazilian agricultural landscapes using carbon and nitrogen stable isotope analyses of hair collected from fecal samples (N = 64). We classified the samples into three groups: feeding patterns based on forest remnants, on the agricultural matrix or both. We observed a predominance of consumption of C₃ prey (~47% of individuals) in the area with the highest proportion of forest coverage. Conversely, C₄ prey were highly consumed (~40% of individuals) where the agricultural matrix was predominant. The δ¹³C values for pumas in both areas indicated that their food resources come from both forest remnants and the agricultural matrix and that some individuals preferentially consumed C₄ prey, indicating that food resources from the agricultural matrix make up most of their diet (~46% of prey individuals). The wide range of puma's δ¹⁵N values in both areas indicated a diet based on different types of prey. However, the C₄ group had higher values, indicating that both pumas and their prey feed on enriched resources from the agricultural matrix. The results confirm the high behavioral plasticity of pumas in using highly anthropogenic habitats. The stable isotope analyses conducted in this study yielded new information on large carnivore trophic ecology that might be useful in the development of new conservation strategies in disturbed areas.     

Assessing trophic interactions between pelagic predatory fish by gut content and stable isotopes analysis around Fernando de Noronha Archipelago (Brazil), Equatorial West Atlantic

The objective of this study was to analyse the feeding habits and trophic interactions between four oceanic predatory fish around the Fernando de Noronha Archipelago (FNA), Brazil, in the western equatorial Atlantic (3.86°S/32.42°W), internationally recognized as an environment of high economic and ecological value. For this purpose, biological samples of yellowfin tuna (Thunnus albacares), wahoo (Acanthocybium solandri), barracuda (Sphyraena barracuda) and dolphinfish (Coryphaena hippurus) were collected for stomach content and stable isotope analysis. Values of the index of relative importance revealed varied diets, with a strong presence of teleost fishes (Diodontidae and Exocoetidae) for all species, with yellowfin tuna having a greater diversity of food items. Despite being generalists/opportunists, the feeding strategy of these predators showed a tendency towards a specialized diet in the use of the available resources around the FNA. They presented a narrow trophic niche width (Levin's index, Bi < 0.6) and low overlap between species, except between barracuda and wahoo (MacArthur and Levin's, R₀ = 0.72). Isotopic compositions had broad values of δ¹³C and δ¹⁵N, and were significantly different between species. Our results provide information about the four species' trophic organization and suggest that the predators avoid competition by preying on different prey, thus allowing their coexistence.     

Climate change and trophic interactions in model temporary pond systems: the effects of high temperature on predation rate depend on prey size and density

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Effects of the presence of a predatory fish and the phenotype of its prey (a shredding shrimp) on leaf litter decomposition

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Sex and occupation time influence niche space of a recovering keystone predator

Predators exert strong effects on ecological communities, particularly when they re‐occupy areas after decades of extirpation. Within species, such effects can vary over time and by sex and cascade across trophic levels. We used a space‐for‐time substitution to make foraging observations of sea otters (Enhydra lutris) across a gradient of reoccupation time (1–30 years), and nonmetric multidimensional scaling (nMDS) analysis to ask whether (a) sea otter niche space varies as a function of occupation time and (b) whether niche space varies by sex. We found that niche space varied among areas of different occupation times. Dietary niches at short occupation times were dominated by urchins (Mesocentrotus and Strongylocentrotus spp; >60% of diets) in open habitats at 10–40 m depths. At longer occupation times, niches were dominated by small clams (Veneroida; >30% diet), mussels (Mytilus spp; >20% diet), and crab (Decapoda; >10% diet) in shallow (<10 m) kelp habitats. Diet diversity was lowest (H′ = 1.46) but energy rich (~37 kcal/min) at the earliest occupied area and highest, but energy poor (H′ = 2.63, ~9 kcal/min) at the longest occupied area. A similar transition occurred through time at a recently occupied area. We found that niche space also differed between sexes, with bachelor males consuming large clams (>60%), and urchins (~25%) from deep waters (>40 m), and females and territorial males consuming smaller, varied prey from shallow waters (<10 m). Bachelor male diets were less diverse (H′ = 2.21) but more energy rich (~27 kcal/min) than territorial males (H′ = 2.54, ~13 kcal/min) and females (H′ = 2.74, ~11 kcal/min). Given recovering predators require adequate food and space, and the ecological interactions they elicit, we emphasize the importance of investigating niche space over the duration of recovery and considering sex‐based differences in these interactions.     

Feeding strategy of silverside species in eutrophic Lake Pátzcuaro,Mexico

The trophic guild, niche breadth, and trophic position of two different ontogenetic phases were measured according to the size of Chirostoma spp. captured in different zones of Lake Pátzcuaro, Mexico. Analyses were carried out at the genus level due the difficulties to find diagnostic characters to separate the three small silverside species accurately, since even the molecular analyses of mitochondrial DNA for a subsample of the most morphologically differentiated specimens were conducted without success. Fish were captured with a seine net during the wet (September and November 2009) and dry (February and June 2010) seasons at four environmentally different sites. Gut contents of 480 Chirostoma specimens were divided into size‐class intervals of 38–51 mm and 66–71 mm for analyses. The contribution of each food item was quantified using the frequency of occurrence and area percentage. Fish were categorized by size, and diets were compared between fish sizes and sites. Dorsal muscle tissue and zooplankton were obtained for nitrogen isotope signature (δ¹⁵N) analyses. The trophic guild was determined using a modified quadrant method for gut content analysis; the importance of each prey item was estimated according to a modified index of relative importance. Niche breadth was calculated by the Levins’ Index. The trophic position was assessed using the TrophLab Program and stable isotope analysis. The index of relative importance among study sites and fish size classes was determined by a multivariate approach. Silversides represent a secondary and tertiary consumer in Lake Pátzcuaro that feed mainly on evasive prey, such as copepods (55%) and on non‐evasive prey, such as cladocerans (25%) at sites with the lowest transparency in the water column (from 350 mm to 150 mm). Fish scales (11%) and insects (6%) were secondary food items. Ingestion of juvenile silversides was rare (1%). Chirostoma spp. prey on insects on the lake bottom and on water hyacinth roots. Based on δ¹⁵N analysis, the trophic position of silversides changes between wet and dry seasons. Their opportunistic and specialized predatory feeding strategy indicates the capability of silversides to resist anthropic changes in the lake.     

Rescaling the trophic structure of marine food webs

Measures of trophic position (TP) are critical for understanding food web interactions and human‐mediated ecosystem disturbance. Nitrogen stable isotopes (δ¹⁵N) provide a powerful tool to estimate TP but are limited by a pragmatic assumption that isotope discrimination is constant (change in δ¹⁵N between predator and prey, Δ¹⁵N = 3.4‰), resulting in an additive framework that omits known Δ¹⁵N variation. Through meta‐analysis, we determine narrowing discrimination from an empirical linear relationship between experimental Δ¹⁵N and δ¹⁵N values of prey consumed. The resulting scaled Δ¹⁵N framework estimated reliable TPs of zooplanktivores to tertiary piscivores congruent with known feeding relationships that radically alters the conventional structure of marine food webs. Apex predator TP estimates were markedly higher than currently assumed by whole‐ecosystem models, indicating perceived food webs have been truncated and species‐interactions over simplified. The scaled Δ¹⁵N framework will greatly improve the accuracy of trophic estimates widely used in ecosystem‐based management.